HID: appletb-kbd: fix slab use-after-free bug in appletb_kbd_probe

In the Linux kernel, the following vulnerability has been resolved:

HID: appletb-kbd: fix slab use-after-free bug in appletb_kbd_probe

In probe appletb_kbd_probe() a "struct appletb_kbd *kbd" is allocated
via devm_kzalloc() to store touch bar keyboard related data.
Later on if backlight_device_get_by_name() finds a backlight device
with name "appletb_backlight" a timer (kbd->inactivity_timer) is setup
with appletb_inactivity_timer() and the timer is armed to run after
appletb_tb_dim_timeout (60) seconds.

A use-after-free is triggered when failure occurs after the timer is
armed. This ultimately means probe failure occurs and as a result the
"struct appletb_kbd *kbd" which is device managed memory is freed.
After 60 seconds the timer will have expired and __run_timers will
attempt to access the timer (kbd->inactivity_timer) however the kdb
structure has been freed causing a use-after free.

[ 71.636938] ==================================================================
[ 71.637915] BUG: KASAN: slab-use-after-free in __run_timers+0x7ad/0x890
[ 71.637915] Write of size 8 at addr ffff8881178c5958 by task swapper/1/0
[ 71.637915]
[ 71.637915] CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.16.0-rc2-00318-g739a6c93cc75-dirty #12 PREEMPT(voluntary)
[ 71.637915] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[ 71.637915] Call Trace:
[ 71.637915] <IRQ>
[ 71.637915] dump_stack_lvl+0x53/0x70
[ 71.637915] print_report+0xce/0x670
[ 71.637915] ? __run_timers+0x7ad/0x890
[ 71.637915] kasan_report+0xce/0x100
[ 71.637915] ? __run_timers+0x7ad/0x890
[ 71.637915] __run_timers+0x7ad/0x890
[ 71.637915] ? __pfx___run_timers+0x10/0x10
[ 71.637915] ? update_process_times+0xfc/0x190
[ 71.637915] ? __pfx_update_process_times+0x10/0x10
[ 71.637915] ? _raw_spin_lock_irq+0x80/0xe0
[ 71.637915] ? _raw_spin_lock_irq+0x80/0xe0
[ 71.637915] ? __pfx__raw_spin_lock_irq+0x10/0x10
[ 71.637915] run_timer_softirq+0x141/0x240
[ 71.637915] ? __pfx_run_timer_softirq+0x10/0x10
[ 71.637915] ? __pfx___hrtimer_run_queues+0x10/0x10
[ 71.637915] ? kvm_clock_get_cycles+0x18/0x30
[ 71.637915] ? ktime_get+0x60/0x140
[ 71.637915] handle_softirqs+0x1b8/0x5c0
[ 71.637915] ? __pfx_handle_softirqs+0x10/0x10
[ 71.637915] irq_exit_rcu+0xaf/0xe0
[ 71.637915] sysvec_apic_timer_interrupt+0x6c/0x80
[ 71.637915] </IRQ>
[ 71.637915]
[ 71.637915] Allocated by task 39:
[ 71.637915] kasan_save_stack+0x33/0x60
[ 71.637915] kasan_save_track+0x14/0x30
[ 71.637915] __kasan_kmalloc+0x8f/0xa0
[ 71.637915] __kmalloc_node_track_caller_noprof+0x195/0x420
[ 71.637915] devm_kmalloc+0x74/0x1e0
[ 71.637915] appletb_kbd_probe+0x37/0x3c0
[ 71.637915] hid_device_probe+0x2d1/0x680
[ 71.637915] really_probe+0x1c3/0x690
[ 71.637915] __driver_probe_device+0x247/0x300
[ 71.637915] driver_probe_device+0x49/0x210
[...]
[ 71.637915]
[ 71.637915] Freed by task 39:
[ 71.637915] kasan_save_stack+0x33/0x60
[ 71.637915] kasan_save_track+0x14/0x30
[ 71.637915] kasan_save_free_info+0x3b/0x60
[ 71.637915] __kasan_slab_free+0x37/0x50
[ 71.637915] kfree+0xcf/0x360
[ 71.637915] devres_release_group+0x1f8/0x3c0
[ 71.637915] hid_device_probe+0x315/0x680
[ 71.637915] really_probe+0x1c3/0x690
[ 71.637915] __driver_probe_device+0x247/0x300
[ 71.637915] driver_probe_device+0x49/0x210
[...]

The root cause of the issue is that the timer is not disarmed
on failure paths leading to it remaining active and accessing
freed memory. To fix this call timer_delete_sync() to deactivate
the timer.

Another small issue is that timer_delete_sync is called
unconditionally in appletb_kbd_remove(), fix this by checking
for a valid kbd->backlight_dev before calling timer_delete_sync.